Electret microphones typically include a permanently charged electret and back plate that are capacitively coupled to provide an output signal to an integrated preamplifier that reflects the magnitude of coupling, and in turn, is indicative of the acoustic signals incident upon the microphone. Due to their relatively simple construction, electret microphones offer low profile and compactness advantages. Further, due to relatively recent developments, electret microphones may be produced at a relatively low cost, while also realizing enhanced performance.
In the later regard, a limiting factor for increasing the signal to noise ratio in a typical electret microphone is the presence of a leakage current that passes through an integrated pre-amplification transistor. The leakage current results from an applied voltage across a semi-conductor channel to a gate of the transistor. Such transistors often utilize silicon oxide insulators between the channel and the gate. However, due to imperfections in the oxide and tunneling effects, leakage currents are still encountered, thereby resulting in undesired noise in the output signal. As may be appreciated, such noise presents particular challenges in hearing instrument applications, and even more particularly, in implanted microphone implementations.